As a part of the U.S. Geological Survey South Florida Ecosystems Initiative and Placed Based Systems Programs, a study was initiated to describe the hydrodynamic characteristics of selected estuarine streams receiving water from the Shark River Slough drainage basin. The analysis of 1999 discharge data provides information on annual discharge characteristics and the effects of weather systems on discharges for the Broad, Harney, and Shark Rivers. These three estuarine-river sites were selected using the criterion that a large amount of the water that flows through Shark River Slough, sometimes referred to as the "Heart of the Everglades," must pass by these sites. Each station was equipped with instruments for recording water level, velocity, specific conductance, and temperature. More recently, the network of monitoring stations has been expanded to cover a larger area, from Whitewater Bay to Everglades City, and includes the North River in White Water Bay, Lostmans and Chatham Rivers to the northwest, and additional sites equipped only with water-level, specific conductance, and temperature sensors. All data generated through this study will be used to describe the salinity patterns in relation to freshwater inflow and weather events along the southwestern coast of Everglades National Park (ENP) and for the development and calibration of the Tides and Inflows in the Mangrove Ecotone (TIME) hydrologic model currently in development by the USGS.

Discharges from the Broad, Harney, and Shark Rivers are influenced by semi-diurnal tides, wind events, and freshwater inflow. All three rivers are well mixed, with a difference in specific conductance from top to bottom usually no greater than 500 microsiemens per centimeter during flood and ebb tides. Discharge is one-dimensional except for brief (less than 20 minutes) periods during slack water (between flood and ebb tide) when flow is vertically bidirectional (moving upstream and downstream). The flood discharges (water moving upstream denoted as negative values) are usually of greater magnitude and shorter duration than the ebb discharges (water moving downstream denoted as positive values).

Instantaneous and residual discharges for the three stations were calculated for the 1999 calendar year. During 1999, the Broad River instantaneous discharges ranged from -2,400 to +3,500 cubic feet per second, whereas the Harney and Shark River instantaneous discharges ranged from -15,600 to +12,900 cubic feet per second and -10,100 to +10,500 cubic feet per second, respectively. The instantaneous discharges values were processed using a ninth-order Butterworth low-pass filter to remove semidiurnal tidal frequencies that eliminates bias associated with lunar cycles when computing daily, weekly, monthly, or yearly mean or median residual (filtered) discharge values. The residual discharges for the Broad, Harney, and Shark River stations ranged from -900 to +2,500 cubic feet per second, -3,600 to +5,700 cubic feet per second, and -2300 to +4,400 cubic feet per second, respectively. The Broad River station is the farthest upstream from the Gulf of Mexico (9.3 river miles) and exhibits less magnitudes of instantaneous and residual discharges than the other two stations and longer duration positive discharges than the Harney (4.4 river miles upstream) or Shark (6.2 river miles upstream) River stations.

Mean annual residual discharges were computed for the Broad and Shark River stations and estimated for the Harney River station. Discharge data were missing for the Harney River from April 4 to June 11, 1999, due to erroneous index-velocity data. This period coincided with prolonged minimum residual discharges recorded at the Broad and Shark River stations. The mean annual residual discharge for the Broad and Shark River stations, using the complete 1999 record were computed as +400 and +440 cubic feet per second, respectively. Excluding the period of missing discharge data for the Harney River station, the mean annual residual discharges for the Broad, Harney, and Shark River stations were +520, +580, and +550 cubic feet per second, respectively. Applying the same difference of about 100 cubic feet per second between mean annual residual discharges for the Broad and Shark River stations, the Harney River station mean annual residual discharge for 1999 was estimated to be about +470 cubic feet per second. The mean annual residual discharges reflect the net downstream flows with minimal errors associated with water storage.

Wind events such as cold fronts, tropical storms, and hurricanes can amplify, attenuate, or completely overwhelm the tidal forces that normally dominate flow patterns in the estuaries along the southwestern coast of ENP. Four strong cold fronts occurred between January and March 1999 that significantly affected short-term discharges (less than a few days) for the Broad, Harney, and Shark Rivers. The lowest water levels for the Broad, Harney, and Shark Rivers occurred during the passage of strong cold fronts in February and March 1999, when mean water levels were lower than in the late summer and early fall. The most significant effects on maximum water level and discharge occurred during the passages of Tropical Storm Harvey and Hurricane Irene in September and October 1999, respectively. The two storms had different effects on the water levels and discharges during their movement toward and away from the southwestern coast.

Tropical Storm Harvey approached the Broad, Harney, and Shark Rivers from the northwest and moved to the east with maximum sustained winds of 60 miles per hour. The winds associated with Harvey forced water into the mangrove forests of the southwestern coast to water levels of about 1.81 feet above sea level at the Broad River station, 3.30 feet above sea level at the Harney River station, and 2.96 feet above sea level at the Shark River station. Some pulsations in water level and discharge, not attributable to semidiurnal tidal forcing, preceded the storm by 2 to 3 days. The center storm surge caused a prolonged flood flow that lasted almost 24 hours; then as the winds shifted and abated, the stored water flowed back to the Gulf of Mexico for about 24 hours with no tidal flow reversal. The maximum positive and negative instantaneous and residual discharges for the Harney and Shark River stations were recorded on September 21, 1999, as Tropical Storm Harvey made landfall. The Broad River discharges exhibited similar patterns, but to a magnitude less than the Harney and Shark River stations due to the location of the station and the storm track.

Hurricane Irene caused a different response at the three river stations because of the storm path and wind strength. Hurricane Irene approached from the southwest and moved to the northeast on October 15, 1999, with maximum sustained winds of 85 miles per hour. The winds associated with Irene forced water from the mangrove forests, and the return seiche was less in magnitude than during Tropical Storm Harvey. Water levels during Hurricane Irene decreased and caused a rapid increase in ebb flow (toward the Gulf of Mexico) that lasted about 24 hours with no flow reversals during the 24-hour period. The Broad River instantaneous and residual discharges reached maximum values of +3,500 and +2,500 cubic feet per second, respectively, during the passage of Hurricane Irene.

(This abstract was taken from "Programs and Abstracts - 2001 Florida Bay Science Conference". (PDF, 6.8 MB))

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